Paper based or paper backed wall coverings are in common use and they are stuck to the wall or other substrate using an adhesive.
The adhesive can be bought as a ready made paste or as a powder which is dissolved in cold water which is then applied with a brush or roller. This method is time consuming, it is difficult to get an even coating of adhesive of the correct thickness for optimum performance and it requires a high degree of skill if the adhesive is to be prevented from staining the decorative surface.
To overcome these disadvantages manufacturers are now supplying prepasted wall coverings with the adhesive applied at a pre-determined optimum coating weight. The adhesive is activated by immersion in water until the adhesive has adsorbed sufficient water to produce the required slip and pliability.
Several types of adhesive are in common use all based on high molecular weight synthetic polymers, e.g. acrylate polymers and co-polymers, or modified natural polymers, such as carboxymethyl cellulose, dextrin and starches.
Modified starches have the major proportion of the market as they can be applied at high solids level (35-40%) and they have reasonably good slip and open (i.e., workable) time. Their disadvantage is the high coating weight (10-20 g/m.sup.2) that must be applied for Vinyl wall coverings and also that such coatings are quite impermeable to air.
Carboxylated high molecular weight acrylic copolymers can be supplied as high solids (40%) and low viscosity (10 cps) emulsions. As such they can be applied to paper at low coating weights and the coated paper can be readily dried. The water insoluble polymer can be activated into an adhesive having good slip and open time by treatment with alkali so that the pH is above 7.5. The alkali can be applied from the soaking bath, but this is considered undesirable due to the lack of control over obtaining the correct pH. To overcome this problem the paper is precoated with an alkaline solution to provide the correct amount of alkali when the paper is immersed in water.
This means two carefully controlled coatings are required. Moreover, the continuity of the coherent coating provided can lead to curling of the paper under certain humidity conditions and prevents the passage of air. Continuity is also an important factor when adhesive backed papers are subsequently coated with polyvinyl chloride (PVC) plastisols because the air in the paper, if it is not able to pass through the adhesive layer comes out through or is entrapped below the PVC causing undesirable blisters.
A further method that has been employed is to electrostatically deposit dry powders, such as carboxymethyl cellulose, on to the wall covering. This technique has associated health and safety hazards and does not produce a well bonded coating. The adhesive coating weight is high and the adhesive coating is likely to dust off.